Methods and apparatus for a flexible circuit interposer

ABSTRACT

A flexible circuit interposer includes a flexible circuit substrate which allows in-situ probing of an attached device during, for example, circuit debugging, assembly qualification, and the like. In accordance with one embodiment, a first set of pads (for example, bond pads configured to interface with solder balls) are configured in a predetermined pattern on the bottom surface of a flexible substrate. Similarly, a second set of pads are configured in substantially the same pattern on the top surface of the flexible substrate, wherein each of the pads in the second set of pads is electrically continuous with a corresponding pad in the first set of pads. A third set of pads are configured in the same pad pattern on the top surface of the flexible substrate. One or more conductive traces are formed to connect one or more pads in the first set of pads with spatially-corresponding pads in the third set of pads. A fourth set of pads corresponding to the third set of pads may also be provided on the bottom surface of the flexible substrate.

FIELD OF THE INVENTION

The present invention relates, generally, to semiconductor devicetesting and, more particularly, to a flexible circuit interposer.

BACKGROUND OF THE INVENTION

Recent advances in semiconductor fabrication techniques havedramatically increased the density and speed of semiconductor devices,leading to a corresponding effort in the field of semiconductorpackaging, where increased device density gives rise to many challengesrelated to electrical connectivity, heat-transfer, manufacturability,and the like. In this regard, a major trend in semiconductor packagingis toward low-profile, high-density device packages such as the variousball grid array (BGA) and fine ball grid array (FBGA) packages.

This increase in device density also poses problems for board-leveltesting, assembly qualification, debug, and other such test procedures.Modem printed circuit boards (PCBs) and modules typically include alarge number of devices in a relatively small space. As a result, it isoften difficult or impossible to electrically probe individual devices(particularly BGA devices) during testing, as the required bond pads orcontacts are not typically exposed for easy access.

Known methods of facilitating device testing include the insertion of“interposers” between the device and board. These interposers typicallyinclude a series of probe contacts provided on one or more sides of thedevice under test. Space limitations in high-density assemblies prohibitthe use of such large, rigid interposers to provide probing capability,as surrounding devices often interfere with optimal placement of suchinterposers. Furthermore, this class of interposer may detach from theboard or reflow during subsequent attachment of the semiconductor deviceto the interposer pads.

Moreover, known interposers are unsatisfactory in that they provide asingle probe point for each terminal. This limitation reduces testflexibility and makes it extremely difficult to determine wherecontinuity issues may exist, for example, open circuits resulting frommechanical overstress of conductive traces formed within the interposer.

SUMMARY OF THE INVENTION

A method and apparatus for a flexible circuit interposer according tovarious aspects of the present invention includes a flexible circuitsubstrate which allows in-situ probing of an attached device during, forexample, circuit debugging, assembly qualification, and the like. Inaccordance with one embodiment, a first set of pads (for example, bondpads configured to interface with solder balls) are configured in apredetermined pattern on the bottom surface of a flexible substrate.Similarly, a second set of pads are configured in substantially the samepattern on the top surface of the flexible substrate, wherein each ofthe pads in the second set of pads is electrically continuous with acorresponding pad in the first set of pads. A third set of pads areconfigured in the same pad pattern on the top surface of the flexiblesubstrate. One or more conductive traces are formed to connect one ormore pads in the first set of pads with spatially-corresponding pads inthe third set of pads. A fourth set of pads corresponding to the thirdset of pads may also be provided on the bottom surface of the flexiblesubstrate.

In accordance with one aspect of the present invention, the first set ofpads are preferably configured to interface with a printed circuitboard, and the third set of pads are preferably configured to interfacewith a semiconductor device (for example, a ball grid array (BGA)device). Thus, the second set of pads (and/or the fourth set of pads)may be probed while the semiconductor is in an operational state. Theflexibility of the interposer allows the device to be advantageouslypositioned within the test apparatus.

BRIEF DESCRIPTION OF EXEMPLARY DRAWINGS

Additional aspects of the present invention are evident upon reviewingthe non-limiting embodiments described in the specification and theclaims, in conjunction with the accompanying figures, wherein likenumerals designate like elements:

FIG. 1 is side view diagram illustrating a flexible circuit interposerin accordance with the present invention attached to a printed circuitboard and a semiconductor device;

FIG. 2 is a top view of an exemplary flexible circuit interposer inaccordance with one embodiment of the present invention;

FIG. 3 is a cross-sectional diagram of various components that may beused in constructing a flexible circuit interposer in accordance withthe present invention;

FIG. 4 is a cross-sectional diagram showing a through-hole providedwithin a flexible circuit interposer in accordance with the presentinvention; and

FIGS. 5A and 5B show an exemplary interposer attached to a board invarious orientations.

Elements in the figures are illustrated for simplicity and clarity andhave not necessarily been drawn to scale. For example, the dimensions ofsome of the elements in the figures may be exaggerated relative to otherelements to improve understanding of embodiments of the presentinvention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Various aspects and features of the present invention may be describedin terms of functional components and steps. Such functional componentsand steps may be realized by any number of elements and/or stepsconfigured to perform the specified functions. For example, the presentmethods and apparatus may employ electronic, signaling, and logicelements which may carry out a variety of functions in variousembodiments, applications, and environments. In addition, the presentmethods and apparatus may be practiced in conjunction with any number ofprocedures and systems, and the apparatus and methods described aremerely exemplary applications for the invention. Further, the presentmethods and apparatus may employ any number of packaging techniques,conventional or otherwise, for placement, use, manufacturing, and thelike.

FIG. 1 shows a flexible circuit interposer 103 in accordance withvarious aspects of the present invention disposed in one possiblecontext—that is, interposed between a semiconductor device 102 and aboard 106. Interposer 103 includes a set of pads 110 that are configuredin a predetermined pattern (for example, a ball-grid array pattern) onthe bottom surface 132 of a flexible substrate 104. Similarly, a secondset of pads 108 are configured in substantially the same pattern on thetop surface 130 of flexible substrate 104, wherein each of the pads inthe second set of pads 108 is electrically continuous with acorresponding pad in the first set of pads 110 (linked, for example, bythrough-holes as described further below).

A third set of pads 118 is configured in the same pad pattern on the topsurface 130 of flexible substrate 104. One or more pads in the first setof pads 110 are electrically continuous with one morespatially-corresponding pads in the third set of pads 118. For example,pad 110(a) may be electrically continuous with pad 118(a), pad 110(b)may be electrically continuous with pad 118(b), and so on. A fourth setof pads 120 corresponding to the third set of pads (and electricallycontinuous with those pads) may also be provided on bottom surface 132of flexible substrate 104.

In accordance with one aspect of the present invention, the first set ofpads 110 are preferably configured to interface with a board 106 viacorresponding solder balls 112 (or other appropriate bonding method),and the third set of pads 118 are preferably configured to interfacewith a semiconductor device (for example, a ball grid array (BGA)device) via corresponding solder balls 116. Thus, the second set of pads108 (and/or the fourth set of pads 120) may be electrically probed whilethe semiconductor is in an operational state. This allows testing of thedevice in-situ.

Semiconductor device 102 may be fabricated using any suitablesemiconductor material and may comprise any convenient packageconfiguration. Suitable materials for device 102 include, for example,group IV semiconductors (i.e., Si, Ge, and SiGe), group III-Vsemiconductors (i.e., GaAs, InAs, and AlGaAs), and otherless-conventional materials, such as SiC, diamond, and sapphire. Thedevice may comprise single crystal material, a silicon-on-insulatormaterial (SOI), or one or more polycrystalline or amorphous epitaxiallayers formed on a suitable base material. It will be appreciated thatdevice 102 will also include various electronic components incorporatedinto the semiconductor material as well as interconnect structuresconsisting of conductive paths and various dielectrics for isolatingthese conductive paths. Such electronic components and processingmethods are well known and therefore will not be discussed in detailherein. In this regard, it will be appreciated that the presentinvention is not limited to a particular class of electronic components.That is, semiconductor device 102 may include any combination of digitaland/or analog semiconductor devices, including, for example,microprocessors, microcontrollers, application specific integratedcircuits (ASICs) static or dynamic memory devices, integrated opticdevices, integrated sensors, and field-effect transistor powersemiconductors. The present invention may be used in connection with awide variety of packages, e.g. ball-grid arrays (BGAs), chip-scalepackages (CSPs) thin quad flat-packs (TQFPs), thin small outlinepackages (TSOPs), and any other package having a plurality of leads orcontacts.

Depending upon the application, board 106 may include rigid boardmaterials (e.g., FR-4 and BT), ceramics, polyimide flex circuits,metallic leadframes or any other suitable material. In one embodiment,board 106 may also include a pattern of conductors between the variouspads 114, and might therefore include a multilevel metallization schemewhich accommodates the conductor/bond pad topology.

Interposer 103 is “flexible” in the sense that it undergoessubstantially elastic deformation in bending when subjected toreasonable strain values. In one embodiment, for example, interposer 103can bend to approximately 70–90 degrees without appreciable damage tothe conductive traces provided therein.

The flexibility of interposer 103 allows the device to be advantageouslypositioned within the test apparatus. That is, referring to FIG. 1,interposer 103 generally bends at approximately its midsection (the“bending region”, as discussed below) such that device 102 may be raisedand/or lowered to avoid other components in the vicinity of device 102,for example, region 150 above board 106, which might be populated by oneor more other components. In this way, interposer 103 allows for gradualtransition away from board 106, and provides a scheme for testingdevices configured in a tight pitch. Interposer 103 may also bepositioned such that device 102 is generally located above and incontact with another component located in region 150.

With reference to the isometric views shown in FIGS. 5A and 5B (which,for the purposes of simplicity, do not show the respective pads),interposer 103 may be positioned, for example, “into” board 106 (FIG.5A), thus extending over one or more adjacent devices 500, or “out of”board 106 (FIG. 5B), thus extending over the side of board. Interposer103 may be attached to the board in any appropriate orientation.Moreover, multiple interposers may be attached to board 106 such thatone or more of the interposers overlap or become “stacked” on top ofeach other.

Referring now to the top view diagram shown in FIG. 2, pads 108 may beconfigured in a pattern 202 on one end of interposer 103, and pads 118may be configured in a pattern 206 on an opposite end of interposer 103.A bending region 204 devoid of pads may be provided between therespective sets of pads. Although a 44-pin pattern is shown in FIG. 2,the present invention is not so limited, and may be used in connectionwith higher pin-count devices, including 65-pin, 144-pin, and higherpin-count devices.

As mentioned above, one or more conductive traces may be provided to mapa subset of pads 108 with a spatially-corresponding pads 118. The phrase“spatially-corresponding” is intended to refer to the position of thepads within their respective patterns (i.e., patterns 202 and 206). Forexample, as shown in the illustrated embodiment, pad 108(c) is connectedto pad 118(c) via a trace 212, and pad 108(d) is connected to pad 118(d)via a trace 210. Each of these pairs of pads are in the substantiallythe same location with respect to the pad pattern, which, in theillustrated embodiment, is a rectangular array having four columns andeleven rows. Those skilled in the art will recognize, however, that thepresent invention may be used in connection with any pad patterncurrently known or developed in the future.

While the embodiment shown in FIG. 2 shows only a portion of respectivepads connected by conductive traces, it may also be advantageous toconnect all or nearly all of the respective pads together. In such acase, however, as discussed further below, it may be desirable to usemultiple-layers of conductive traces to avoid collisions between traceswithin a given layer.

In accordance with one aspect of the present invention, patterns 202 and206 correspond to rectangular arrays of pads having an aspect ratio ofbetween approximately 2:1 and 4:1, preferably about 3:1. In accordancewith another aspect of the present invention, bending region 204 issuitably about the width of pattern 204. The present invention is not solimited, however, and comprehends any suitable combination of pad aspectratio and bending region dimensions.

Referring now to the cross-sectional (see A′ in FIG. 2) diagram shown inFIG. 3, the present invention may be fabricated using a flexible,conductor-clad middle section 304 sandwiched between two coveringoverlays (or “coverlays”) 302 and 306.

In one embodiment, middle section 304 includes a polymeric layer 314(for example, polyimide) clad with thin copper layers 312 and 316. Thecopper layers 312 and 316 may be formed using any convenient technique,for example, through thermal-compression bonding of copper foil (e.g.,0.5 oz. copper foil). A number of materials are suitable for middlelayer 304, including, for example, any of the various copper-cladlaminates manufactured by Dupont, such as PYRALUX Model Number AP8525double-sided copper-clad polyimide. The thickness of section 304 mayalso be selected in accordance with, for example, the desiredflexibility. In one embodiment, for example, the thickness of section304 is approximately 50.0 microns.

An appropriate conductive trace pattern connecting one or more spatiallycorresponding pads may be formed using layer 312, layer 316, or both.Furthermore, it may be desirable to use three, four, or even more layersto form the conductive traces. Formation of the conductive traces may beaccomplished using a variety of conventional techniques, for example,using standard wet or dry photolithographic techniques.

Coverlay 302 suitably includes a flexible layer 308 (for example,polyimide) and an adhesive layer 310 (for example, an acrylic or epoxyadhesive). Similarly, coverlay 306 suitably includes a flexible layer320 and an adhesive layer 318. Adhesive layer 310 acts to bond layer 308to layer 312 (and exposed areas of layer 314), and adhesive layer 318acts to bond layer 320 to layer 316 and any exposed areas of layer 314.

A variety of materials are suitable for coverlays 302 and 306,including, for example, any of the various flexible composite coverlaysmanufactured by Dupont, such as Dupont's PYRALUX Model FR7001 laminate.The thickness of coverlays 302 and/or 306 may be selected in accordancewith the application. In one embodiment, the coverlays have a thicknessof approximately 25.0 microns.

Referring now to the cross-sectional (see A′ in FIG. 2) diagram shown inFIG. 4, a through-hole 402 may be formed between conductive layer 312and conductive layer 316. If coverlays are to be used as describedabove, through-holes 402 may be formed after application of thecoverlays.

Through-hole 402 includes a conductive wall 404 and, optionally, pads406 on opposite sides of interposer 103. Through holes 402 may have anysuitable dimensions depending upon the application. In the illustratedembodiment, the through-holes have an as-plated diameter ofapproximately 0.008 inches. In one embodiment, the walls 404 comprise acopper layer no less than approximately 0.001 inches thick, and pads 406comprise a gold layer (for example, an electroplated gold layer having athickness of from about 3–10 micro-inches) over a nickel layer (forexample, a layer of low-stress nickel having a thickness ofapproximately 300 micro-inches).

In one embodiment, pads 406 are square pads having approximately 0.010inch sides. It will be appreciated, however, that any suitable shape maybe used for pads 406. The through-holes 402 are positioned to intersectwith the appropriate pads 406 to provide the desired electricalconnectivity. In addition, the pads may be enlarged in certain regionsto accommodate variations in through-hole placement and thus increasethe manufacturability of the interposer. For example, in one embodimenta “tear-drop” pattern is added to conductive traces at the pads.Additional information regarding the manufacturing of through-holes andother conventional packaging techniques may be found in a number ofstandard texts, e.g., Seraphim, Lasky, and Li, PRINCIPLES OF ELECTRONICPACKAGING (1989).

In summary, methods and apparatus in accordance with the presentinvention provide an improved, flexible circuit interposer which allowsin-situ probing of an attached device during, for example, circuitdebugging, assembly qualification, and the like.

The present invention is described with reference to various preferredembodiments. However, changes and modifications may be made to thevarious exemplary embodiments without departing from the scope of thepresent invention. These and other changes or modifications are intendedto be included within the scope of the present invention as set forth inthe appended claims.

1. A flexible circuit interposer comprising: a substantially flexiblesubstrate having a top surface and a bottom surface; a first set of padsconfigured in a first pad pattern on said bottom surface of saidflexible substrate; a second set of pads configured in a second padpattern on said top surface of said flexible substrate, wherein saidsecond pad pattern is substantially the same as the first pad pattern,and wherein each of said pads in said second set of pads is electricallycontinuous with a corresponding pad in said first set of pads; a thirdset of pads configured in a third pad pattern on said top surface ofsaid flexible substrate, wherein said third pad pattern is substantiallythe same as the first pad pattern; at least one conductive traceconnecting a pad in said first set of pads with a spatiallycorresponding pad in said third set of pads; and a bending regionbetween said first set of pads and said second set of pads.
 2. Theflexible circuit interposer of claim 1, wherein said first set of padsis configured to interface with a printed circuit board.
 3. The flexiblecircuit interposer of claim 2, wherein said third set of pads isconfigured to interface with a semiconductor device.
 4. The flexiblecircuit interposer of claim 3, wherein said third set of pads isconfigured to interface with a ball grid array (BGA) device.
 5. Theflexible circuit interposer of claim 1, wherein said second set of padsis configured to interface with one or more test probes.
 6. The flexiblecircuit interposer of claim 1, further comprising a fourth set of padsconfigured in a fourth pad pattern on said bottom surface of saidflexible substrate, wherein said fourth pad pattern substantiallycorresponds to said third pad pattern, wherein each of said pads in saidfourth set of pads is electrically continuous with a corresponding padin said third set of pads.
 7. The flexible circuit interposer of claim6, wherein said fourth set of pads is configured to interface with oneor more test probes.
 8. The flexible circuit interposer of claim 1,wherein said flexible substrate includes a polymer layer.
 9. Theflexible circuit interposer of claim 8, wherein said polymer layercomprises polyimide.
 10. The flexible circuit interposer of claim 8,wherein said polymer layer has a thickness of between approximately 25.0and 75.0 microns.
 11. The flexible circuit interposer of claim 1,wherein said flexible substrate includes at least one coverlay layerover said at least one conductive trace.
 12. The flexible circuitinterposer of claim 11, wherein said coverlay layer comprises a polymerfilm layer and an adhesive layer.
 13. The flexible circuit interposer ofclaim 1, wherein at least a portion of said pads comprise a layer ofnickel.
 14. The flexible circuit interposer of claim 1, wherein at leasta portion of said pads comprise a layer of gold.
 15. The flexiblecircuit interposer of claim 1, wherein said first set of pads and saidsecond set of pads are connected by plated through-holes.
 16. Theflexible circuit interposer of claim 1, wherein said pad pattern is arectangular array.
 17. The flexible circuit interposer of claim 16,wherein said rectangular array has an aspect ratio of betweenapproximately 2:1 and 4:1.
 18. The flexible circuit interposer of claim1, wherein said bending region has a width approximately equal to awidth of said first set of pads.
 19. The flexible circuit interposer ofclaim 1, wherein said pad pattern has a pitch of between approximately0.5 and 1.5 microns.
 20. The flexible circuit interposer of claim 1,wherein said bending region has a width that is approximately x times apitch of said pad pattern, and where x is between approximately 3.5 and4.5.